In fabricating circuit boards, a coating is usually applied to certain portions thereof either to mask them temporarily during subsequent processing or to protect them permanently.
Temporary masking is typically effected by selectively coating the concerned portions of the circuit board with a maskant type coating material that is stripped from the board at a later fabrication stage. Permanent protection is typically effected by coating the concerned portions of the circuit board with a conformal type coating material that conforms to the surface configuration of the board to seal therewithin pertinent circuit components of various shapes and heights on the board that are to be protected. In both cases, the coating is usually subjected to curing to impart structural integrity thereto.
U.S. Pat. No. 4,254,163 (Piazza) shows a thermographic printing method for making a given type printed circuit on a metal clad substrate. Resin powder is dusted onto a pattern of non-dryable, hydrophobic and heat non-spreadable, liquid ink on the substrate, the powder adhering only to the ink. Heat fusing the powder to the ink forms a strippable mask which is stripped to form the circuit after etching the ink-free substrate areas.
U.S. Pat. No. 4,287,226 (Pirvics et al.) shows the curing of an ultraviolet (UV) light curable resin to form a protective coating on a given type circuit element-containing substrate. The resin is exposed to UV light through a silk screen having opaque areas that mask the UV light from corresponding areas of the resin that are not to be cured. Assumably, the non-cured resin areas of the coating are thereafter stripped.
U.S. Pat. No. 4,826,705 (Drain et al.) shows the mechanical stripping of a laminated portion of a conventionally cured conformal coating and underlying ultraviolet light cured temporary mask that cover an edge connection area on a given type printed circuit board, thereby exposing the area. The cured mask is readily stripped in one piece in the manner of adhesive tape.
U.S. Pat. No. 4,830,922 (Sparrowhawk et al.) shows use of a solvent removable coating of glass microspheres in a grease as a temporary protective conformal coating for selective components of various shapes and heights on a given type circuit board.
U.S. Pat. No. 4,623,559 (Hudock) shows an assembly line conveyor for dipping a given type thermistor in a bath of ultraviolet (uv) light curable coating material, and then passing each thermistor under uv light to cure the coating.
U.S. Pat. No. 4,695,482 (Weiswurm) shows an assembly line conveyor for programmed top and bottom side coating and curing of two different types of circuit boards horizontally clipped thereto. At a sole station, a sensor stops the conveyor on sensing a board, and the appropriate one of two identifiers determines the type board sensed. This causes the appropriate one of two types of upper nozzle sets to descend to the board for selectively spray coating its top side. At the same time, its bottom side is grossly coated by an underlying wave coating unit. The nozzle set is then raised and the conveyor restarted for sensing and spraying the next board in time-consuming sequential tandem steps at the sole station. The coated boards continue on the conveyor for intermittent travel through an oven for curing.
U.S. Pat. No. 3,836,388 (Fowler) shows a manually operated batch apparatus for applying a silicone rubber coating on a given type circuit-containing substrate, in sets of four substrates at a time. Each set of coated substrates is then transferred by the operator to the next available one of a plurality of cam actuated vacuum degassing chambers on an adjacent manually operated rotary table (dial), after the operator removes a previously degassed set of coated substrates from that chamber for curing elsewhere.
U.S. Pat. No. 4,661,368 (Rohde et al.) shows a program controlled spray nozzle movable to apply adhesive dots to a given type circuit board for holding later added components.
U.S. Pat. No. 4,560,584 (Henninger) shows a program controlled spray nozzle movable to apply individual dots of liquid solder resist material onto selected portions of a given type circuit board to mask such portions and prevent them from being coated in a subsequent wave soldering step. The resist material is then removed in a cleaning step.
U.S. Pat. Nos. 4,753,819 and 4,880,663 (Shimada) commonly show a program controlled spray nozzle movable to apply a flowable, e.g., conformal, coating material selectively onto predetermined portions of a circuit board having circuit components of various shapes and heights. Airless spraying technique is used to coat such portions without overspraying onto adjacent board portions not intended to be coated.
Nordson Corporation (Nordson Electronics Business Group, Amherst, Ohio), Product Data Sheet 306-18-857, issued February 1989, shows a local or an in-line, program controlled, spray nozzle conformal coating system for selectively coating a given type circuit board by a robotic unit available in 4-axis and 5-axis configurations.
Integrated Technologies, Inc. (Acushnet, Mass.), Product Data Sheets, Series S5000 and CL7000, also show program controlled spray nozzle conformal coating systems for selectively coating a given type circuit board by robotic units.
None of these prior art teachings involves the uninterrupted performing of respective sequential operations simultaneously at successive stations of a multiple station assembly line, selectively on randomly differing types of workpieces in individually differing orientation, respectively at the stations.
It is desirable to provide a system for performing work on workpieces of different types in random order in assembly line fashion, wherein the system is flexible in that it enables work to be performed selectively on each workpiece, regardless of its type, orientation or assembly line order, without interrupting on-line processing to adjust the system to each type workpiece.